# Electronic – Clamping Constant Current LED Driver Output Voltage

I want to test a Constant Current LED Driver across its voltage range without using a LED for test purpose.

How can I clamp the output voltage of this driver to required voltage levels?
For Example –
LED Driver Ratings – 800mA Constant Current Voltage Range – 17V to 34V.
Now how do I test this driver at 17V, 18V etc to make sure this driver works under different voltages.

I also tried to clamp the voltage using an Op-amp but in vain. I am not sure if output voltage is stiff enough to clamp LED driver output voltage. Also, current is getting drawn from the output of Opamp whereas opamp is primarily used to just clamp the voltage.

Any thoughts on how this can be achieved?

Set your electronic load to resistance mode and adjust the synthetic resistance to get the desired voltage drop.

I think this is probably best general answer for almost all of those with commercial electronic loads. All I have used have constant current, constant resistance and constant power modes. That's the whole point of such devices- to act as flexible loads- most will also allow you to simulate changing loads etc. If yours does not for some reason, keep reading:

If yours does not, you can use a shunt regulator based on an op-amp with a PNP darlington (and a reference voltage) or use the ciruit in the TL431 data sheet, substituting a PNP darlington (eg. TIP125) for the PNP transistor and choosing the B-E resistor so that the TL431 always conducts at least 1mA.

The resistor in series with Vi is not required in this application- the series impedance of the current source takes its place (and should be quite high in dynamic resistance if it is a good constant current source). Vref for the TL431 is about 2.5V (2.495V nominally). Depending on the discrepancy between the load and source the transistor could see a lot of dissipation (18V * 0.8A = 14.4W if the load was completely disconnected). That would require a large heatsink. If the load is set to a higher current than the source, the output voltage will collapse which may cause your source to misbehave, or worse.

If your "load" is just a current sink (CC mode only) it is not very appropriate for this purpose). You might as well just use the above circuit alone with a suitable heatsink.